Although I've come up with an even more ambitious design that incorporates jet engines, enabling it to take off from a runway, ascend to 120,000 feet above sea level, then transition to rocket-powered flight to get into orbit. Upon landing it can use the jets to maintain better control and to allow the pilot to make multiple attempts at landing.
And Nasa won't have to spend money on building giant external fuel tanks, refurbishing solid rocket boosters, or even maintaining the crawler transporters. They can just roll the plane out to the runway and have it blast off from there!
Though I could actually have it built only with support from somebody other than NASA. The most likely candidate is the US Air Force if it has combat capability (I've been designing it with ASAT capability in mind)